Surface-controlled, subsurface safety valves (SCSSVs) are commonly used to shut-in oil and gas wells. The SCSSV fits onto production tubing in a well and operates to block flow of formation fluid upwardly through the tubing should a failure or hazardous condition occur at the well surface. The SCSSV can be tubing retrievable and rigidly connected to the production tubing (tubing retrievable), or it can be wireline retrievable and installed and retrieved by wireline without disturbing the production tubing.
Most SCSSVs are “normally closed” and use a flapper type closure mechanism biased to a closed position. A hydraulic actuator can be moved longitudinally in the SCSSV to overcome the flapper's bias and open the valve. Typically, the actuator uses a piston and a flow tube.
During normal production, hydraulic pressure transmitted to the piston moves the flow tube longitudinally in the valve to keep the flapper open. The hydraulic pressure is commonly supplied by a control line run along the annulus between the production tubing and casing. When a hazardous condition occurs, the SCSSV provides automatic shutoff of the production flow. The hazardous condition can be sensed and/or indicated at the surface or elsewhere and can include a fire on the platform, a high/low flow line pressure condition, a high/low flow line temperature condition, operator override, or the like.
Once the condition is sensed or indicated, the hydraulic pressure is removed from the control line, and the loss of hydraulic pressure causes the flapper to close and block the flow of production fluids up the tubing. When the flapper closes (as well as opens), the flapper's mating surface engages with the flow tube. In fact, the conventional flapper has a concentrated area on its inside surface that engages with the flow tube as they both moving during closing (or opening). This area and even the flapper's sealing surface can be damaged or deformed during harsh opening and closing operations.
The direct solution to address the problem of damage to the flapper simply involves limiting the flow level for which the flapper mechanism is rated. Alternatively, the flapper's thickness can be increased to make it more robust, but this reduces the cross-sectional flow area that can pass through the valve. In any event, operators strive for valves providing as much flow area as possible when open and capable of operating in high working pressures. When operators need a valve with a very slim diameter, such as 7-in., addressing problems with damage to the flapper becomes even more problematic.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.